This invention relates to molding compositions for making lightweight fiberglass reinforced parts which can be molded at low pressures in molds made of less expensive materials than the hardened steel tools needed in conventional compression molding.
Sheet molding compound (SMC) is made from unsaturated thermosetting resins, a shrink control additive, a monomer reactive with the thermoset resin, an alkaline earth oxide or hydroxide thickening agent which forms ionic bonds with acid functionality in the resin, a mold release, an inert filler and a free radical initiator. These ingredients are applied to a chopped glass fiber mat, compacted and rolled up or festooned between cover sheets or barrier films made of polyethylene or nylon. The formed sheet is typically stored for 2 to 4 days to allow the paste component to thicken. This thickening process is also referred to as xe2x80x9cbuildingxe2x80x9d viscosity or xe2x80x9cmaturingxe2x80x9d the compound. The cover sheet is peeled off of the mature sheet to yield a tack free molding compound having a viscosity of about 15-50 million centipoise. The tack free compound is loaded into a heated press and molded at temperatures of 85xc2x0 C.-170xc2x0 C. under pressures of 5.5 MPa to 8.0 MPa for 0.5 to 4.0 minutes.
Large presses and costly tools are required to process conventional sheet molding compounds. According to MODERN PLASTICS (March, 1994), compression molding compound usually requires pressures of 800 psi (5.5 MPa) or more to mold parts. To accommodate these high molding pressures, hard steel tools made of P20 or 4140 grade steel are required. These tools are expensive to make relative to cast steel or composite tools. If compounds could be molded at lower pressures, less expensive tools and smaller less expensive presses could be used.
One type of low pressure molding compound is described in the U.S. Pat. No. 5,075,393 assigned to Scott Bader Company where crystalline unsaturated polyester resins which are solid at ambient temperatures, are used instead of chemical thickeners to provide the handling properties which, in conventional sheet molding compound, are provided by the ionic bonding of alkaline earth oxides with acid functionality in the resin. This use of resins which are solid at ambient temperatures requires compounding at elevated temperatures, approximately 93xc2x0 C. to 100xc2x0 C.
In attempts to provide resins which are moldable at low pressures, various resins have been tried which, when thickened with alkaline earth oxides, reach lower matured viscosities. Such resins behave poorly when shrink control additives are used, the molding sheets are too tacky and sticky for operators to cut and handle.
It is therefore an object of this invention to develop a molding resin composition capable of molding at pressures less than 3.5 MPa. A further object of this invention is to develop a resin which can be compounded at ambient temperatures.
Another object of this invention is to develop a resin composition having good handling characteristics when in sheet form, so that the sheets are not tacky to the touch and the cover sheet or film removes readily.
A further object of this invention is to provide a sheet molding composition which, when molded into automobile parts, meets the exacting criteria required by car manufacturers such as those set forth by Chrysler Motors Corporation, Ford Motor Company and General Motors Corporation.
It has been discovered that light weight molded parts can be molded at low pressures from a molding composition containing a thixotrope such as fumed silica and, optionally, filler. The process of making the molded part of this invention includes the steps of
a) admixing thermosetting resin, shrink control additive, monomer, thixotrope, such as fumed silica, free radical initiator, mold release, alkaline earth oxide or hydroxide thickening agent,
b) forming a paste,
c) dispensing said paste above and below a bed of chopped roving glass,
d) forming a molding sheet,
e) consolidating said sheet,
f) enveloping said sheet in a carrier film,
g) maturing said sheet until it reaches a viscosity of 3 million centipoise to 18 million centipoise measured with a Brookfield 5X HBT viscometer (TF spindle, 5 rpm, Model D Heliopath) and said sheet is non-tacky,
h) releasing said sheet from said film, and
i) molding said sheet at a temperature of 85 to 170xc2x0 C. and at a pressure less than 500 psi (3.5 MPa) whereby a uniform flow of resin and glass occurs outward to the edges of said part.